Single-crystal X-ray studies of four representatives of "pushpull" proton sponges, namely 1,8-bis(dimethylamino)-, 1,8-bis(diethylamino)-, 1,8-bis(dipropylamino)-4,5-dinitronaphthalenes and 4,5-bis(dimethylamino)naphthalene-1,8-dicarbaldehyde have been performed at low and ambient temperatures. The most interesting and unexpected result is that the formyl groups in the peri-dialdehyde display stronger π-acceptor effects than the nitro groups. This phenomenon is ascribed to smaller steric demands of the CHO groups, their lower electrostatic repulsion, and specific packing forces.
Perimidones, 2,3-dihydroperimidines [2, 3] and perimidines [4] are known to be easily acylated at the 6(7)-(para) or 4(9)-(ortho) positions. Carboxylic acids in polyphosphoric acid were used as the acylating agents. For perimidones, acyl chlorides in the presence of anhydrous AICI3 were used [5]. Therefore, we expected that formylation of these compounds might proceed just as easily. The present work tests that hypothesis.We found that 1,3-dialkylperimidones (Ia and Ib) undergo the Vilsmeier reaction (POC13-DMF) at 60-80~ to form the perimidon-6-carboxaldehydes Ha, b in 80-85% yield. It seemed interesting to determine if the monoaldehydes could be further formylated, keeping in mind that perimidones readily undergo diacylation [3]. Heating IIa with an excess of Vilsmeier reagent (60-80~ 5.5 h) led to formation of resin. The only product in about 20% yield was 1,3-dimethylperimidon-4-carboxaldehyde (III), which was somewhat unexpectedly isolated. A similar isomerization was previously observed for 6-acylperimidines with an unsubstituted N-H group [4]. It was proposed that this isomerization is driven by the formation in 9-acyl derivatives of a strong intramolecular hydrogen bond (IHB). Since this is not a factor for aldehyde III, the migration of the formyl group to the more sterically hindered ortho position is apparently due to some other reason. It is noteworthy that aldehyde IIa is completely destroyed by heating with polyphosphoric acid for -30 min, forming a dark green crystalline product. Judging from the PMR spectrum, the product is a complex mixture of oligomers. The tendency to oligomerize in acidic medium probably explains the relatively low yield oflII. Formylation of 1,3-dimethyl-2,3-dihydroperimidine (IV) is more complicated and significantly less regioselective. The reaction goes slowly even at -20-(-30)~ However, it is better conducted at room temperature.
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